My map page sometimes breaks into the pixilated image shown. Refreshing the screen always clears the problem. I make the pfd full screen size then pull up the map again to resolve the issue.
The purple are areas where ADS-B data hasn't yet been received. It fills in over minutes as you get the nexrad data blocks all around you. But, are you saying that turning the layer off and on produces an instant dramatic change?
This sometimes happens as new data is transmitted- so it may just be that in the time you switch the MAP off then on, the full update comes through. Next time it happens, wait a minute and see if it resolves itself.